System, Methods And Apparatus For Waking An Autonomous Active Implantable Medical Device Communicating By Pulses Transmitted Through The Interstitial Tissues Of The Body

ABSTRACT

An autonomous active medical implantable device, with a power supply and a wake-up circuit that responds to receipt of specific pulses transmitted through the interstitial tissues of the body transmitter device ( 40 ) generates trains of modulated pulses applied to electrodes ( 22, 24 ), and a receiver ( 50 ) processes (e.g., filter, amplify and demodulate) pulses collected on electrodes ( 22′, 24 ′). The receiver circuits ( 50 ) are selectively activated from a dormant (sleep) state in which they are not powered by a power source ( 34 ), to an operational (active) state in which they are powered and able to process (e.g., filter, amplify and demodulate) the collected pulses specific wake-up pulse train, configured in a predetermined characteristic pulse pattern triggers passive wake-up circuits ( 66 ) in the receiver ( 50 ) to switch the receiver circuits from the sleep state to the operational state.

RELATED APPLICATIONS

The present application claims the benefit of French Application No.10/58476 entitled “Autonomous active medical implant, with a wake-upcircuit of the power supply on receipt of pulses transmitted through theinterstitial tissues of the body” and filed Oct. 18, 2010, which ishereby incorporated by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to “medical devices” as defined by the 14Jun. 1993 Directive 93/42/EEC of the Council of the EuropeanCommunities, including the “active implantable medical devices” asdefined by the 20 Jun. 1990 Directive 90/385/EEC of the Council of theEuropean Communities, more particularly, to devices that continuouslymonitor a patient's cardiac rhythm and deliver if necessary to thepatient's heart electrical pulses for stimulation, resynchronization,cardioversion and/or defibrillation in response to a cardiac rhythmdisorder detected by the device, and to neurological devices, cochlearimplants, etc., as well as devices for pH measurement or devices forintracorporeal impedance measurement (such as the measure of thetranspulmonary impedance or of the intracardiac impedance). The presentinvention is even more particularly directed to those implantabledevices that are autonomous capsules free from any physical connectionto a main implanted device (such as the can of a stimulation pulsegenerator) or a main not-implanted device (e.g., an external device suchas a programmer or other device for remote monitoring of the patient),which capsules are considered to be a “remote device” relative to a“main” device, and communicate with the main device or another remotedevice by intracorporeal communication of signals via the interstitialtissues of the body, which is called human body communication (“HBC”).

BACKGROUND

Autonomous implanted capsules are often called “leadless capsules” anddistinguished from those electrodes or sensors placed at the distal endof a lead, which lead is traversed throughout its length by one or moreconductors, connecting by galvanic conduction, the electrode or sensorto a generator connected at the opposite, proximal end, of the lead.Such leadless capsules are, for example, described in U.S. PatentPublication No. 2007/0088397 A1 and WO 2007/047681 A2 (Nanostim, Inc.)or in the U.S. Patent Production No. 2006/0136004 A1 (EBR Systems,Inc.).

Leadless capsules may be epicardial capsules fixed to the outer wall ofthe heart, or endocardial capsules fixed to the inside wall of aventricular or atrial cavity. Their attachment to the heart wall isusually achieved by a protruding anchor such as a helical screw axiallyextending from the body of the capsule and designed to screw into andpenetrate the heart tissue at the implantation site.

Leadless capsules typically include detection and/or stimulationcircuitry to collect depolarization potentials of the myocardium and/orto apply stimulation pulses to the site where the leadless capsule isanchored, respectively. Such leadless capsules include an appropriateelectrode, which optionally can be included in an active part of ananchoring screw. It can also incorporate one or more sensors for locallymeasuring the value of a parameter that is characteristic of thepatient's physiological or physical condition, such as, for example, thepatient's oxygen level in the blood, endocardial cardiac pressure, heartwall acceleration, and acceleration of the patient as an indicator ofphysical activity.

It should be understood, however, that the present invention is notlimited to a particular type of leadless capsule, and is equallyapplicable to any type of leadless capsule, regardless of its functionalpurpose.

Of course, for a leadless capsule to exchange data with a remote device(i.e., in this context a device that is “remote” to the leadlesscapsule), the leadless capsule incorporates transmitter/receiver meansfor unidirectional and/or bidirectional wireless communications, asdeemed appropriate for the purpose of the leadless capsule. Severaltechniques have been proposed for such wireless communications, inparticular to allow a remote device to centralize the informationcollected by the leadless capsule and send, if necessary, appropriateinstruction controls signals to the leadless capsule. As noted, theremote device in this instance may be an implanted pacemaker,defibrillator or resynchronizer, a subcutaneous defibrillator, or along-term event recorder, and may be implanted or not implanted, and maybe called a main device.

Thus, U.S. Patent Publication No. 2006/0136004 A1 proposes to transmitdata by acoustic waves propagating inside the body. This technique issafe and effective, but it nevertheless has the disadvantage ofrequiring a relatively high transmission power, given the attenuation ofacoustic waves into the body, and allows only relatively low datatransmission rates.

U.S. Pat. No. 5,411,535 proposes a communication technique based on theuse of radiofrequency (RF) waves. This also requires relatively hightransmission power, and the attenuation of these waves by intracorporealtissue is a major barrier to their spread.

Another technique proposed by U.S. Pat. No. 4,987,897 is a data exchangewith a remote external device (programmer), through the skin, ratherthan only an intracorporeal transmission. This transmission is over arelatively short distance, between on the one hand, the housing of apacemaker implanted in a subcutaneous pocket and, on the other hand, anexternal programmer placed against the skin near the generator. Currentstherefore circulate through the skin in an area very distant from thesensitive areas, particularly in an area very distant from themyocardium, which avoids any risk of disruption of the natural orstimulated depolarization waves of the myocardium.

U.S. Patent Publication No. 2007/0088397 A1 proposes to use thestimulation pulses produced by a leadless capsule as a vehicle for thetransmission of data previously collected or created by the leadlesscapsule. To this purpose, the pulse, instead of presenting a monotonicvariation of voltage, is interrupted in a controlled manner for veryshort durations in order to create in the profile of the pulse verynarrow pulses whose sequence corresponds to binary encoding of theinformation to be transmitted.

Whatever the communication technique used, the processing of the HBCsignal collected at the leadless capsule requires significant energycompared to the energy resources available in the leadless capsule.Given its autonomous nature, the leadless capsule can in fact only useits own power resources, such as an energy harvester circuit (based onthe movement of the leadless capsule) and/or a small integrated battery.The management of the available energy is thus a crucial point for thedevelopment of HBC techniques with and between autonomous leadlesscapsules.

It is recognized that the communication between a leadless capsule and aremote device is not continuous, and the active circuitry for signalprocessing (e.g., one or more of signal conditioning, amplification,scanning, filtering, decoding, and signal analysis) is thereforeunnecessarily powered, with a negative impact on the autonomy of theleadless capsule and hence its useful lifetime, in the absence of aspecific sleep implementation. By a “sleep implementation” it is meantthat the relevant active circuits are powered down or off to conserveenergy consumption (generally referred to as “sleep” or a “sleep mode”).

On the other hand, if the active circuits of the receiver are in a sleepmode then it becomes impossible to detect the occurrence of a HBCsignal.

There is thus a need to “wake-up” the leadless capsules from a sleepmode when needed, with a relatively low latency to avoid burdening theflow of HBC signals or the responsiveness of the leadless capsule to thecontrol signals that it receives by this HBC technique.

One known wake-up technique is to keep the active receiver circuits in asleep implementation and wake them up at regular intervals to detect thepresence of signals sent by another leadless capsule or another remotedevice. This technique requires a compromise between a low frequency ofwake-ups—which saves energy but reduces the responsiveness—andconversely a higher frequency—which improves responsiveness but onlyslightly reduces the average energy consumption of the capsule in thelong term.

US Patent Publication No. 2008/071328 A1 proposes an implantableleadless device equipped with a wake-up circuit activated by RF oracoustic signals (thus, not HBC signals) emitted from another remotedevice, these signals being possibly coded according, for example, to aparticular addressing scheme allowing the wake-up of only one particular(or more) selected device. The use of non-HBC signals for the wake-up,however, requires the implantation of specific communication circuitsboth on the receiver and on the transmitter side of the communication.

The present invention, however, provides a different approach. It isbased primarily on the use of a clock circuit that is a passive receiver(that is to say, a wake-up circuit that operates without an amplifier orany other circuit component having a significant energy consumption ascompared to the overall energy balance of the leadless capsule)constantly able to detect the reception of a HBC signal. Only when suchan HBC signal has been detected, does the clock circuit wake-up thesystem receiver circuit and activate the amplifiers and the other activecircuits that need energy.

One of the major difficulties of this approach has been the lowamplitude level of HBC signals to be detected by the wake-up circuit,given the rapid attenuation of propagating electrical pulses in theinterstitial tissues of the body.

Another difficulty is that, the pulses of HBC signals can be mixed withparasitic electrical signals of relatively high level, such as thebody's natural myopotentials and the cardiac depolarization wavespropagating within the myocardium—hence there is a deterioratedsignal/noise ratio. The stimulation pulses delivered by an implantedgenerator should also be discriminated so as to exclude them, whichstimulation pulses are locally applied at specific sites of themyocardium, but then diffuse around the site of stimulation in a largearea before being substantially attenuated.

It is desirable that the implemented technique should be able to addressthese issues and ensure effective screening and discrimination of allspurious signals.

As regards more particularly the signal attenuation by body tissues inthe frequency range 500 kHz-10 MHz (band B), which is the one withminimal attenuation by the interstitial tissues of the body, theattenuation in this frequency range typically varies between 10 dB and40 dB, depending on the distance between the transmitter and thereceiver, the distance between the respective electrodes of the pair ofelectrodes of the transmitter or of the receiver, and the surface ofthese electrodes. A typical attenuation value is 20 dB at 1 MHz for adistance of 10 to 12 cm between the transmitter and the receiver.

But in extreme conditions, this attenuation can reach 60 dB, so that theclock signal sent to the leadless capsule is mixed in a backgroundnoise, thus having both a very low pulse amplitude to be detected and avery poor signal/noise ratio.

OBJECT AND SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to preserve theuseful life of a leadless capsule, dependent on an integratedself-powered system. It is another object to provide a leadless capsulethat is not likely to be affected by spurious electrical signals presentin the body tissues, including the disruption by cardiac myopotentialsand other potentials present in the body, or by the stimulation pulsesthat may be delivered by an implanted device. It is yet another objectof the present invention to have a leadless capsule that does notrequire complex circuits for a sleep implementation and has minimallatency in response to a wake-up pulse, to ensure a high responsivenessof the leadless capsule.

One embodiment of the present invention is directed to a system of thegeneral type disclosed in US Patent Publication No. 2008/071328 A1 citedabove, namely, including at least two active implantable medical devicesthat communicate with each other intracorporeally by electrical signalsconsisting of electrical pulses conducted by the interstitial tissues ofthe body (HBC signals). The system includes at least one device actingas a transmitter device and at least one device acting as a receiverdevice, each device being remote from the other. The transmitter devicecomprises a transmitter that generates modulated pulse trains having aseries of HBC signal pulses that are applied to electrodes of thetransmitter device. The transmitter, prior to or at the start of acommunication sequence with the receiver device, also generates aspecific wake-up signal, configured according to a predeterminedcharacteristic pattern. The wake-up signal may or may not be modulatedby the transmitter device along with the pulse train of informationbeing communicated to the receiver device. The receiver device includeselectrode terminals to receive the HBC signals and receiver circuitsthat process, e.g., condition, filter, amplify and demodulate signalpulse trains, the HSC signals collected at the receiver device electrodeterminals. These receiver circuits are selectively activated from asleep implementation, wherein they are not powered by a power source ofthe receiver device, to an operational or awake state, wherein they arepowered for, e.g., filtering, amplifying and demodulating the collectedpulses.

The receiver device thus includes, in addition to the receiver circuits,a clock circuit (also referred to herein as a “wake-up circuit”) thatdiscriminates the reception of a specific wake-up signal, ascorresponding to the predetermined characteristic pattern of thespecific wake-up signal emitted by the transmitter and, in response,switches the receiver circuits from the sleep state to the active oroperational state.

In one embodiment, the wireless communication is an intracorporealcommunication of HBC signals constituted by said electrical pulses,conducted by the interstitial tissues of the body; the said specificwake-up signal generated by said transmitter is a train of said pulsesconfigured according to a predetermined characteristic pulse pattern;and the clock circuit means includes detector means and an hysteresiscomparator.

In one preferred embodiment, the wake-up circuit is advantageouslydevoid of an amplifier circuit, and may in particular comprise ahysteresis comparator. The permanent energy consumption of the wake-upcircuit is typically less than 10 nW.

In one embodiment, the pulses of the specific wake-up pulse train arepreferably biphasic pulses, each including positive and negativealternations.

In a first embodiment of the invention, the transmitter generates thepredetermined characteristic pattern in the form of an initial pulse ofhigh amplitude for the wake-up signal, and then generates a followingsuccession of pulses of lower amplitude for communication of desiredinformation to the receiver device. The characteristic pattern mayinclude a period between the initial pulse of high amplitude and thesuccession of pulses of lower amplitude, selected to set the wakeuplatency of the receiver device.

In one embodiment, the wake-up circuit preferably comprises an up-streamhigh-pass filter for mitigation of parasitic physiological signalspresent in the interstitial tissues of the body, a detector circuit thatis preferably either a peak level detector or a peak-to-peak detector,of the received pulses, and means for inhibiting the switching of thereceiver circuits from the sleep state to the operational state upondetection of a received pulse having a width greater than apredetermined upper limit.

In one embodiment of the invention, the transmitter generates thepredetermined characteristic pattern of the wake-up signal in the formof a specific sequence of uniform amplitude pulses encoding apredetermined binary code forming a wake-up code. The wake-up circuitinclude circuits to identify the received pulses, to decode the binaryvalue encoded by these received pulses, and compare the decoded valuewith a predetermined value stored in the receiver device to determinewhether the decoded signal correspond to a specific wake-up signal.

In one preferred implementation of the invention, the transmitter meansgenerates the specific sequence of pulses in the form of a plurality ofdistinct bursts of pulses, some of these bursts being composed of afirst number of pulses encoding a binary ‘1’ and the bursts being madeof a second number of pulses, different from the first number, forexample, double, encoding a binary ‘0’. The wake-up circuit thenpreferably includes an envelope detector delivering as an output aseries of binary values each obtained by integration of a receivedcorresponding burst of pulses, and digital circuitry for comparing thedetected series of bits to the predetermined stored value. The envelopedetector may advantageously include a voltage multiplier circuit.

Another aspect of the present invention is directed to a receiver devicefor an autonomous active implantable medical device that has wirelesscommunications with a remote active medical device, wherein the receiverdevice includes an energy source, at least one active circuit elementused in connection with the processing of received signals, a pair ofelectrodes, and a receiver means for collecting a signal at said pair ofelectrodes and processing said received signal to identify therein areceived modulated pulse train. The receiver means includes means forprocessing the received signal, more specifically demodulating areceived modulated pulse train into a received pulse train andidentifying in said received pulse train a plurality of pulsescorresponding to information being transmitted. The wake-up signal ispreferably a biphasic pulse, and the modulated pulse train is preferablya biphasic pulse train, each biphasic pulse having a positivealternation and a negative alternation. The receiver means also includescircuit means for selectively switching the at least one active circuitelement between a sleep state not powered by the energy source and anoperational state powered for processing the collected signal using saidat least one active circuit element. The receiver device also includes awake-up circuit means for discriminating said received signal andidentifying therein a first signal having a predetermined characteristicpulse pattern corresponding to a wake-up signal, and in response,switching the receiver means from the sleep state to the operationalstate. The wake-up circuit means further comprises a detector circuit,which may be one of a peak level detector and a peak-to-peak detector,and a hysteresis comparator. More preferably, the wake-up circuit meanshas a permanent energy consumption of less than 10 nW. In an embodimentwhere the wake-up signal is modulated with the information pulse train,the receiver device first demodulates the received signal and thendiscriminates the wake-up signal.

Preferably, the wake-up circuit means has no active amplifier circuit.

In one embodiment of the receiver, the predetermined characteristicpattern is an initial pulse having a high amplitude corresponding to thewake-up signal, wherein the following succession of pulses have loweramplitude. The predetermined characteristic pattern also may have alatency period between the initial pulse and the following succession ofpulses. Preferably, the wake-up circuit means further comprises meansfor inhibiting the switching of the receiver means from the sleep stateto the operational state upon detection of a received pulse having awidth that is greater than an upper predetermined limit (T). In oneembodiment, the predetermined value corresponding to an authorizedwake-up signal may be a first specific sequence of pulses of relativelyuniform amplitude (A) encoding a predetermined binary value forming awake-up code, and the wake-up circuit means comprises means forindentifying the received signal and decoding therefrom the first signalas a binary value encoded by said received signal first specificsequences of pulses, and comparing said decoded binary value with saidpredetermined value. Further, the first specific sequence of pulses maybe a plurality of distinct bursts of pulses, some of said bursts being afirst number of pulses encoding a ‘1’ bit and the other bursts being asecond number of pulses, different from the first number, encoding a ‘0’bit. More preferably, the second number of pulses is double the firstnumber of pulses. Also, the receiver means preferably includes anenvelope detector delivering as output a series of binary values eachobtained by integration of a received corresponding burst of pulses, andmeans for comparing said series of bits to said predetermined value. Theenvelope detector is preferably a voltage multiplier circuit.

Another aspect of the invention is directed towards a transmittercircuit for an active medical device having wireless communication witha remote active medical device, wherein the transmitter device includesa first pair of electrodes and a transmitter circuit that generates awake-up signal and a following modulated pulse train having a pluralityof pulses in a series, the latter corresponding to a communication ofdesired information to said remote device. The wake-up signal isprovided with a predetermined characteristic pulse pattern andoptionally may be modulated along with the plurality of pulsescorresponding to the information. The generated wake-up signal andmodulated pulse train pulses are applied to the electrodes forcommunication to a remote device. Preferably, the predeterminedcharacteristic pattern has an initial pulse having a high amplitudecorresponding to the wake-up signal, and the following succession ofpulses have a lower amplitude. The wireless communication is preferablyan intracorporeal communication of electrical pulses conducted by theinterstitial tissues of a patient's body.

In one embodiment of the transmitter device, the wake-up signal is abiphasic pulse and the modulated pulse train is a biphasic pulse train,with each biphasic pulse having a positive alternation and a negativealternation.

In one embodiment of the transmitter device, the predeterminedcharacteristic pattern includes a latency period (T_(w)) that extendsbetween the initial pulse and the following succession of pulses.Preferably, the predetermined characteristic pattern further comprises afirst specific sequence of pulses of uniform amplitude encoding apredetermined binary value forming a wake-up code. The wake-up code canbe used by the remote device for authenticating a received signal as awake-up signal, as discussed herein.

In one embodiment of the transmitter device, the first specific sequenceof pulses is a plurality of distinct bursts of pulses, some of saidbursts being a first number of pulses encoding a ‘1’ bit and the otherbursts being a second number of pulses, different from the first number,encoding a ‘0’ bit. More preferably, the second number of pulses isdouble the first number of pulses.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features, characteristics and advantages of the presentinvention will become apparent to a person of ordinary skill in the artfrom the following detailed description of preferred embodiments of thepresent invention, made with reference to the drawings annexed, in whichlike reference characters refer to like elements, and in which:

FIG. 1 schematically illustrates a representative system of medicaldevices including leadless capsules, implanted within the body of apatient;

FIG. 2 schematically illustrates a plurality of leadless capsulesimplanted on the inner and outer walls of a patient's myocardium;

FIG. 3 is a functional block schematic diagram showing the differentcircuit stages constituting a leadless capsule;

FIG. 4 is a schematic circuit diagram of a transmitter circuit and areceiver circuit in accordance with a preferred embodiment of thepresent invention;

FIGS. 5 a and 5 b are timing diagrams showing the shape of a wake-uppulse train according to a first embodiment of the present invention,respectively, as emitted and as received;

FIG. 6 is a schematic circuit diagram of the different circuit stages ofa clock circuit according to an embodiment of the present invention,this circuit being able to detect and analyze a pulse such as that inFIG. 5 b;

FIG. 7 illustrates the shape of two wake-up pulses emitted according toa second embodiment of the present invention;

FIG. 8 is a schematic circuit diagram of the circuit stages of a wake-upcircuit according to the second embodiment of the present invention,circuit being able to detect and analyze the received pulses at thereceiver side of FIG. 7; and

FIG. 9 illustrates, on the same timing diagram, a representative rawwake-up signal, as received, and the corresponding demodulated signalobtained at the output of the envelope detector circuit of FIG. 8.

DETAILED DESCRIPTION OF THE INVENTION

With reference to the drawings FIGS. 1-9, preferred embodiments of asystem, transmitter and receiver, in accordance with the invention willnow be described.

In FIG. 1, a system of medical devices 10, 12, 14, 16 implanted withinthe body of a patient, wirelessly communicating with each other throughHBC signals, is illustrated.

Device 10 which may be, for example, adefibrillator/pacemaker/resynchronizer, or a subcutaneous defibrillatoror a long-term event recorder, is implanted in the patient andconsidered to be a “master” device of a network comprising a pluralityof “slave” devices 12 to 18 with which it is capable of communicating byHBC signals. These slave devices may include intracardiac 12 orepicardial 14 leadless capsules implanted directly onto the patient'sheart, other devices 16 such as myopotential sensors or neurologicalstimulation devices implanted at suitable locations, and possibly anexternal device 18, e.g., disposed on an armband and provided withelectrodes in contact with patient's skin.

Device 10 can also be used as a gateway to the outside world tocommunicate with an external (not implanted) device 20 such as aprogrammer or a remote data transmission device, with which device 10can communicate via RF telemetry in the MICS (Medical ImplantsCommunication System) band 402-405 MHz, or the unmarked public ISM(Industrial, Scientific and Medical) bands 863-870 MHz, 902-928 MHz and2.4 GHz, or such other frequency band as may used by medical devices.

Each of the devices 10 to 18 is provided with at least one pair ofelectrodes which are in direct contact with the tissues of the body(implanted devices 10, 12, 14, 16), or in contact with the skin(external device 18).

With reference to FIG. 2, an example of a leadless capsule implantedeither on the anterior part of the myocardium, within an atrial orventricular cavity (endocardial capsules 12) or on an outer wall of thesame myocardial (epicardial capsules 14) is shown. These capsules, whichare described for example in U.S. Patent Publication No. 2007/0088397A1, WO 2007/047681 A2 and U.S. Patent Publication No. 2006/0136004 A1above, which disclosures are hereby incorporated herein by reference,are attached to the heart wall using a projecting anchoring screw forpenetrating into the cardiac tissue by screwing at the implant site. Thescrew can either be a passive screw, serving only for fixing thecapsule, or an active screw, for collecting the signals ofdepolarization that propagate in the myocardium tissue and/or fordelivering localized stimulation pulses to the implantation site.

FIG. 3 schematically shows the different internal circuits of leadlesscapsules 12, 14 (and, mutatis mutandis, of the other implanted devicesdesigned to communicate with each other via HBC signals in accordancewith the present invention). Each leadless capsule contains a pair ofelectrodes 22, 24, one of which may also be constituted by the anchoringscrew in the heart tissue. These electrodes are connected to astimulation pulse generator circuit 26 (for an active leadless capsuleincorporating this function) and/or a detection circuit 28 for thecollection of depolarization potentials collected between the electrodes22 and 24.

A central processing unit circuit 30 is a microprocessor and/ormicrocontroller based device executing suitable software instructions(CPU) that provides control of the various functions of the leadlesscapsules, including delivery of stimulation pulse (if applicable), andmemorization (storage in memory) of all or selected collected signals.The leadless capsule can also be equipped with a sensor 32 such as, forexample, an acceleration sensor, a pressure sensor, a hemodynamicsensor, a temperature sensor, an oxygen saturation sensor for obtaininga relevant parameter of the patient. The leadless capsule is preferablypowered by souce 34, which may be a small battery or by an energyharvester circuit (or both) supplying all the powered circuits via apower management stage 36.

The electrodes 22 and 24 are also connected to a modulator/demodulatorcircuit 38 coupled to the CPU 30 for emitting and/or receiving pulsesused for wireless HBC signals. Thus, depending on whether thestimulation (module 26) and collection (module 28) circuits are presentor not, the electrodes 22, 24 can provide a single, double or triplefunction, namely, stimulation and/or collection of cardiac potentials(if applicable) and/or transmission of information obtained by thesensor 32 (if necessary) and emission/reception for the HBC signals (inany case).

The present invention improves the power management stage 36, includingthe circuits and methods used to selectively power the activeconsumer-of-energy circuits (e.g., amplifier 28, CPU 30, HBCtransmission/reception circuit 38), only when they are needed. The restof the time, these active circuits are put to sleep, so they do notconsume any power or any significant amount of power.

Power management stage 36 thus includes clock circuitry for this wake-upfunction consisting only of passive components and/or components withvery low power consumption (so as to be equivalent to passive componentsin terms of total energy balance), as described in detail hereafter.

The wake-up circuit has the function of detecting, during the sleep ofthe receiver circuits (i.e., the active circuits), pulse trainsconfigured in a predetermined characteristic pattern specific to triggera wake-up of those active circuits in a sleep mode.

The pulse train that constitute the wake-up signal is remotely generatedby one device such as a leadless capsule that has a pacemaker functionor a subcutaneous device for monitoring cardiac activity and generatingstimulation, resynchronization and/or defibrillation pulses, or byanother leadless capsule of the system. The remote device can, forexample, provide the function of a master device (e.g., a hub in a starwireless network system architecture), with the other various leadlesscapsules in the system being slave devices for receiving a communicationin this one direction from the master device—it being understood thatfor communication in the opposite direction, the roles are reversed).

FIG. 4 schematically illustrates the elements used by the master device(or “transmitter device”) and by the slave leadless capsule to be wokenup (or “receiver device”) in accordance with a preferred embodiment ofthe present invention.

The circuits of the transmitter device 40 includes a current source (orvoltage source) 42, adjustable by command to generate a voltage pulse ofan appropriate value. The CPU 30 controls the opening and closing ofswitches, including the closure of a switch 44 to inject the current ina predetermined direction and for a predetermined time interval. Theinjected current 52 flows (via a coupling capacitor 46, shared or notwith the stimulation stage, to avoid sending any direct voltage on theelectrodes) through the patient's body from one of the electrodes 22 upto the other electrode 24. Switch 48 can then be operated to dischargecapacitor 46 of any residual charge due to the compensation errors ofthe positive and negative pulses. After injecting a first alternation,the procedure is the same to inject the next alternation by reversingthe direction of the current, so as to obtain biphasic pulses, asdescribed below.

Preferably, biphasic pulses are generated to minimize the injectedresidual charge into the patient's heart and also reduce the possiblecorrosion of materials. In the examples shown, these biphasic pulses aredelivered in the form of two successive, positive and negative (or viceversa), alternations, with a square and symmetrical shape (i.e., thesame magnitude in absolute terms for the same two oscillations period).Other waveforms are possible, however, and the examples set forth hereshould be considered as not limiting.

Referring to FIG. 4, reference 50 designates a receiver circuit of thereceiver device: current 52 circulating through the body generatesbetween the electrodes 22′ and 24′ of the receiver a potentialdifference that is applied to an amplifier stage 54 via the connectingcapacitors 56 and 58 to eliminate any DC bias. The resulting amplifiedsignal is applied to a bandpass filter 60 to filter the parasiticsignals outside the relevant band, and the resulting filtered signal isapplied to a threshold comparator 62, along with a reference V_(th), andto a demodulator stage 64.

In accordance with one embodiment of the invention, circuits 54, 62 and64, are active circuits (energy consumers) of the receiver device and,preferably together with any other active circuits, are powered onlywhen necessary to reduce the average energy consumption of the receiverdevice.

In accordance with the present invention, electrodes 22′ and 24′ areconnected to a clock (wake-up) circuit 66 for detecting anddiscriminating the arrival of specific wake-up pulses on theseelectrodes. Wake-up circuit 66 responds to such a wake-up pulse train bydelivering a control wake-up signal S_(w) that is applied to the inputof power management stage 36 to supply the active circuits 54, 62, 64,previously in a sleep state, with the appropriate voltage from powersource 34.

Two specific embodiments of the present invention will now be described,that are specifically adapted to different qualities of thecommunication channel between a given transmitter device and a givenreceiver device, namely: (i) the signal attenuation between thetransmitter and the receiver, and (ii) the signal/noise ratio more orless degraded by interfering signals present in the interstitialtissues.

The first embodiment, illustrated in FIGS. 5 and 6, is optimized forcommunication channels with low attenuation and a good signal/noiseratio. “Low attenuation” should be understood to mean an attenuationbetween ×10 and ×100, or between 20 to 40 dB. In this case, apredetermined characteristic pulse pattern, such as that shown in FIG. 5a, including a (biphasic) pulse 68 of high amplitude A_(w), isgenerated, forming a wake-up signal that is used to trigger the wake-up,followed by a succession of (also biphasic) pulses 70 of smalleramplitude A, for the HBC signals to be delivered to the receiver device.

A biphasic pulse of ±1.5 V (i.e., a peak-to-peak amplitude A_(w)=3 V)can for example be generated at the transmitter device side, whichprovides for an attenuation of between 20 and 40 dB, and a pulse havingan amplitude between about ±150 mV and ±15 mV, at the receiver deviceside.

The received signal corresponding to the transmitted signal iscomparable to that illustrated in FIG. 5 b, which is an attenuated andfiltered (by the tissues of the body) representative version of theoriginal emitted signal. The received signal is thus a pulse trainhaving a first pulse 68′ of relatively higher amplitude and a series ofpulses 70′ of lower amplitude which follow.

With reference to FIG. 6 the different stages of the wake-up circuit 66are shown. Circuit 66 includes a high pass filter 72 receiving theupstream signal collected on the electrodes 22′ and 24′ and deliveringas an output the filtered signal to a detector stage 74. Detector stage74, which can be a simple amplitude detector or, more preferably, apeak-to-peak detector such as that illustrated in FIG. 6. Using apeak-to-peak detector improves the sensitivity of the detection system.The output of detector 66 is applied to the input of a hysteresiscomparator 76, which switching generates the signal S_(w) intended toawake the active circuits of the receiver device.

High pass filter 72 allows filtering the physiological signals so as notto make them visible to the detector and comparator circuits arrangedupstream, and thus avoid false wake-ups. Filter 72 can be a passiveresonant circuit tuned to the frequency of the pulses which amplifiesthe signal between the electrodes, or a simple first order high passfilter made by an RC circuit, the values of R and C being chosen so asto greatly reduce the physiological signals compared the hysteresis ofcomparator 76. The physiological signals to be filtered out areessentially made up of myopotentials of the body and of depolarizationwaves (EGM signals) that propagate through the myocardium, whetherspontaneous or stimulated. For example, a passive first order high passfilter with a cut-off frequency of 1 MHz attenuates of 80 dB the EGMsignals whose spectral content is in the band 0-100 Hz, so that an EGMsignal of 40 mV does not contribute more than 4 μV to the receivedsignal collected on the electrodes 22′ and 24′ signal, which isgenerally of an amplitude of several tens of millivolts.

Amplitude detector 74 is preferably a peak-to-peak detector, configuredas a rectifier consisting of two capacitors and two diodes. The voltageat the output of the detector 74 is equal to V_(max)−V_(min)−2 V_(d),where V_(max) is the maximum of the received signal, V_(min) is theminimum of the received signal after filtering, and V_(d) is thethreshold voltage of the diodes. If this threshold voltage is low, awell known state of the art peak-to-peak detector without losses can beachieved under the name of “active diode” which is implemented usingseveral MOSFET switches in order to avoid the classical voltage drop indiodes.

The threshold V_(s) of the comparator 76 is preferably set to V_(s)=0 V(the comparison level to 0 V simplifies the electronics associated withthe circuit, compared to a comparator with a preset voltage). Thehysteresis value depends on the noise level of reception. In the case ofthe example mentioned above (wake-up pulse generated at a voltage of±1.5 V, with 40 dB attenuation) for a peak-to-peak voltage of 30 mVavailable at the output of the detector 74, the hysteresis of comparator76 can be set, for example, to a value of about 15 mV.

Another spurious signal to be eliminated is that formed by an electricalstimulation pulse applied to the heart, which has a high amplitude thatmay inappropriately trigger wake-up circuit 66. To prevent this untimelytriggering, an additional criterion can be added to validate the wake-upand generate the signal S_(w). This condition is to check (stage 78)that the width t of the logic signal generated at the output ofcomparator 76 is below a predetermined duration T, When T is selected tobe close to the duration of the wake-up signal pulse (which, inpractice, is much shorter than a stimulation pulse).

To test this condition, a first solution is to count the time betweenthe falling edge and rising edge of the received pulse 68′, using acounter (e.g., a clock) preferably already available in the receiverdevice, provided that the cycle of the counter is shorter than theduration of the wake-up signal pulse. If after a certain time limit(time-out falling edge does not occur, the detection is disabled and theS_(w) signal is not generated.

An alternative technique that may be used for validation of wake-upsignal is to integrate the output signal of comparator 76, for example,using a simple RC circuit, and comparing the result to a predeterminedthreshold.

In either case, some time has to pass (the time-out delay) todiscriminate a wake-up signal (short pulse) from a stimulation pulse(long pulse). In practice, a timeout delay period of about three to fourtimes the duration of the wake-up signal pulse 68 is considered to be anacceptable compromise, which does not reduce too much the responsivenessof the wake-up circuit. Of course, other periods can be used.

The signal S_(w) produced by wake-up circuit 66 starts the supply ofpower to the various active circuits of the receiver device, which canthen receive and process for analysis the HBC signals. To reflect thestart time of these receiver circuits to their power on, there is alatency T_(w) (FIG. 5 a) between the wake-up signal pulse 68 and pulseof the HBC signals 70 corresponding to the information to betransmitted.

Furthermore, if the channel quality is good enough to detect HBC signalsof low amplitude without amplification, filtering stage 72, detectionstage 74 and comparator stage 76 of the wake-up circuit 66 can also beused for HBC signal exchange. This reduces the complexity and size ofthe receiver device, using a single circuit for the wake-up of thereceiver and the HBC data signal collection.

Thus wake-up circuit 66 in accordance with the present inventionrequires little energy because the stages 72 and 74 are made with purelypassive circuits. The only power consuming element is the comparator 76,but the current CMOS technologies can allow achieving a continuouslyactive hysteresis comparator with an extremely low average powerconsumption, on the order of tens or hundreds of nanowatts. Bycomparison, an active signal amplifier uses about 1000 times more power.This amount is negligible compared to the overall energy balance of aleadless capsule in its operational state with all its circuits active,which is around 5 to 10 μW.

With reference to FIGS. 7 and 8, a second implementation of the presentinvention will be described, which is particularly suitable forsituations in which the first implementation is not suitable, such ascases of high attenuation (where high attenuation means up to ×1000, 60dB) and/or of significantly degraded signal/noise ratio.

In this embodiment, the proposed wake-up signal is, as shown in FIG. 7,formed from pulse bursts such as 80 or 82, which encode one bit ofbinary signal, respectively ‘0’ or ‘1’. This is an amplitude shift key(“ASK”) type of amplitude modulation, an “all or nothing”, the pulsesbeing biphasic signals of constant amplitude A (peak to peak). For abasic pulse of duration T_(s), a binary zero is encoded by a biphasicburst of n pulses, for example n=3 pulses as shown in FIG. 7, followedby a null signal for the same period n*T_(s). The binary ‘1’ are encodedby a burst of 2n biphasic pulses 82 (n=6 pulses in the illustratedexample), followed by a null signal for 2n*T_(s).

This modulation is in fact a double modulation: in the baseband, themodulation is a pulse width modulation (PWM) in which the length ofzeros is equal to T₀ (n pulses, then no signal, the duration of which isn*T_(s)) and the length of each is equal to T₁=2T₀. The amplitude of theresulting signal (illustrated at the top of FIG. 7) is then modulated inthe baseband by a pattern of pulse bursts to get the two bursts 80 or 82(illustrated at bottom of FIG. 7).

FIG. 8 shows a preferred embodiment of a wake-up circuit 68 to detectand discriminate these pulse bursts. This circuit includes first anenvelope detector stage 84, advantageously comprising a voltagemultiplier 86, followed by a rectifier and a passive low-pass filter 88,e.g., a first order low pass filter RC. In the example shown in FIG. 8,a multiplier-by-four circuit realized with capacitors and low thresholddiodes is shown, such circuit being feasible in standard CMOStechnology. This multiplier has the effect of storing at the outputcapacitor (capacitor of stage 88) terminals a voltage equal to fourtimes the amplitude of the received signal on the electrodes 22′ and24′, thus achieving an amplification of passive voltage (the fact ofhaving an AC signal—the pulse burst—makes it possible to achieve suchamplification without energy consumption).

With reference to FIG. 9, a representative demodulated signal S_(d),obtained at the output of the envelope detector 84, relative to therepresentative very noisy input signal S_(i), collected on theelectrodes 22′ and 24′, are shown.

If the signal from the envelope detector 84 is applied to a decodercircuit 90 including, as in the first embodiment, a hysteresiscomparator 92 receiving at one of its inputs the signal S_(d), thereference voltage applied to other input is advantageously constitutedby a dynamic threshold that is the average value of the signal S_(d),obtained by an RC integrator circuit 94. This prevents the setting ofthe decoder circuit 90 according to the quality of the transmissionchannel (attenuation and signal/noise ratio). In other words, instead ofa pulse of high amplitude as in the first embodiment, in this secondembodiment multiple pulses of uniform amplitude are sent, andintegration of the succession of these pulses is applied to achieve acomparable result.

The decoding of the detected signal is made with no clock,asynchronously, by storing the successive bits received and decoded inan N bit (predefined size) shift register 96. At the end of thereception of the sequence of bits, the content of the shift register iscompared by a comparator 98 to a numerical reference code, stored in aregister 100 of the device. If it matches, the signal S_(w) is generatedto wake-up the active circuits of the receiver device in the same manneras described for the first embodiment. Unlike the first embodiment,however, in this second embodiment there is no need to discriminate acommunication pulse from a stimulation pulse as it is impossible toobtain the reference numerical value of the register 100 for astimulation pulse.

The numerical value stored in the memory 100 may be a generic wake-upcode (e.g., for an undifferentiated wake-up of all the devices in thesystem), or a code specific to each device acting as a receiver, so thatthe master transmitter device can selectively awake and communicate witha particular slave device of the system.

In this second embodiment, the detection quality of the wake-up signaldepends on the duration of a bit and not just of the signal/noise ratio.The number of pulses encoding a bit (n bits to encode a ‘0’ and 2n bitsto encode a ‘1’) can thus be increased and increases with the timeconstant (integration time) of the envelope detector to increase thequality of reception—with a corresponding reduction in the communicationflow.

Again, the circuits of the wake-up circuit of the second embodiment arepassive circuits, with the exception of the comparators 92 and 98, whoseenergy consumption is, however, extremely low, as explained above inconnection with the first embodiment.

One skilled in the art will appreciate the present invention may bepracticed by embodiments other than those disclosed herein, which areprovided for purposes of illustration and not of limitation.

1. A system for wireless communication between at least two activemedical devices in which at least one active medical device is atransmitter device and at least one active implantable medical device isa receiver device, wherein: the transmitter device comprises a firstpair of electrodes and a transmitter means (40) for generating modulatedpulse trains comprising a wake-up plurality of pulses in a seriescorresponding to a communication to the receiver device, and a wake-upsignal preceding said pulse train, said wake-up signal having apredetermined characteristic pulse pattern, and for applying saidwake-up signal and modulated pulse train pulses said to electrodes (22,24); and the receiver device comprises an energy source, at least oneactive circuit element, a second pair of electrodes (22′, 24′), and areceiver means (50) for collecting a signal at said electrodes andprocessing said received signal to identify therein a received wake-upsignal and modulated pulse train corresponding to said wake-up signaland modulated pulse train from said transmitter means, and means fordemodulating said received modulated pulse train into a received pulsetrain corresponding to said plurality of pulses; wherein said receivermeans (50) further comprises means for selectively switching said atleast one active circuit element between a sleep state not powered bysaid energy source (34) and an operational state powered for processingsaid signal using said at least one active circuit element; and, wake-upcircuit means (66) for discriminating said received wake-up signalhaving said predetermined characteristic pulse pattern, and in response,switching the receiver means from the sleep state to the operationalstate, wherein: said wireless communication is an intracorporealcommunication of electrical pulses conducted by the interstitial tissuesof a patient's body; and the wake-up circuit means comprises a detectorcircuit (74, 84) and a hysteresis comparator (76, 88).
 2. The system ofclaim 1, wherein the wake-up circuit means has no active amplifiercircuit.
 3. The system of claim 1 wherein said wake-up signal comprisesa biphasic pulse and said modulated pulse train (68, 70, 80, 82) furthercomprises a biphasic pulse train, each biphasic pulse having a positivealternation and a negative alternation.
 4. The system of claim 1,wherein the wake-up circuit means has a permanent energy consumption ofless than 10 nW.
 5. The system of claim 1, wherein said predeterminedcharacteristic pattern further comprises an initial pulse (68) having ahigh amplitude (A_(w)) corresponding to the wake-up signal, and whereinsaid following succession of pulses (70) have lower amplitude (A) thansaid high amplitude.
 6. The system of claim 5, wherein the predeterminedcharacteristic pattern further comprises a latency period (T_(w))between said initial pulse and the following succession of pulses. 7.The system of claim 5, wherein the receiver means further comprises oneof a peak level detector and a peak-to-peak detector (74), of thereceived signal.
 8. The system of claim 5, wherein the wake-up circuitmeans further comprises means (78) for inhibiting the switching of thereceiver means from the sleep state to the operational state upondetection of a received pulse having a width that is greater than anupper predetermined limit (T).
 9. The system of claim 1, wherein thereceiver means further comprising a predetermined value corresponding toan authorized wake-up signal, said predetermined characteristic patternfurther comprises a first specific sequence of pulses (80, 82) ofuniform amplitude (A) encoding a predetermined binary value forming awake-up code, and the wake-up circuit means comprises means (84-100) foridentifying the received signal and decoding therefrom a binary valueencoded by said received signal first specific sequences of pulses, andcomparing said decoded binary value with said predetermined value. 10.The system of claim 9, wherein the first specific sequence of pulsesfurther comprises a plurality of distinct bursts of pulses, some of saidbursts being a first number of pulses (80) encoding a ‘1’ bit and theother bursts being a second number of pulses (82), different from thefirst number, encoding a ‘0’ bit, and wherein the receiver means furthercomprises an envelope detector (84) delivering as output a series ofbinary values each obtained by integration of a received correspondingburst of pulses, and means (96, 98, 100) for comparing said series ofbits to said predetermined value.
 11. The system of claim 10, whereinthe envelope detector (84) comprises a voltage multiplier circuit (86).12. The system of claim 10, wherein the second number of pulses (80) isdouble the first number of pulses (82).
 13. The system of claim 1wherein said modulated pulse train includes said wake-up pulse precedingsaid plurality of pulses, and the means for demodulating said receivedsignal further comprises means for demodulating said received modulatedpulse train into a received pulse train corresponding to said wake-upsignal and said following plurality of pulses.
 14. An autonomous activeimplantable medical device having wireless communications with a remoteactive medical device, having a receiver device comprising: an energysource; at least one active circuit element; a pair of electrodes (22′,24′), and a receiver means (50) for collecting a signal at said pair ofelectrodes and processing said received signal to identify therein awake-up signal and a received modulated pulse train, said receiver meansfurther comprising means for demodulating said received modulated pulsetrain into a received pulse train and identifying in said received pulsetrain a plurality of pulses; means for selectively switching said atleast one active circuit element between a sleep state not powered bysaid energy source (34) and an operational state powered for processingsaid signal using said at least one active circuit element; and, wake-upcircuit means (66) for discriminating said received signal andidentifying therein a first signal having a predetermined characteristicpulse pattern and corresponding to a wake-up signal, and in response,switching the receiver means from the sleep state to the operationalstate, wherein: said signal further comprises an intracorporealcommunication of electrical pulses conducted by the interstitial tissuesof a patient's body; and the wake-up circuit means further comprises adetector circuit (74, 84) and a hysteresis comparator (76, 88).
 15. Thedevice of claim 14, wherein the wake-up circuit means has no activeamplifier circuit.
 16. The device of claim 14 wherein said wake-upsignal further comprises a biphasic pulse and said modulated pulse train(68, 70, 80, 82) further comprises a biphasic pulse train, each biphasicpulse having a positive alternation and a negative alternation.
 17. Thedevice of claim 14, wherein the wake-up circuit means has a permanentenergy consumption of less than 10 nW.
 18. The device of claim 14,wherein said predetermined characteristic pattern further comprises aninitial pulse (68) having a high amplitude (A_(w)) corresponding to thewake-up signal, and wherein said following succession of pulses (70)have lower amplitude (A) than said high amplitude.
 19. The device ofclaim 18, wherein the predetermined characteristic pattern furthercomprises a latency period (T_(w)) between said initial pulse and thefollowing succession of pulses.
 20. The device of claim 18, wherein thereceiver means further comprises one of a peak level detector and apeak-to-peak detector (74), of the received signal.
 21. The device ofclaim 18, wherein the wake-up circuit means further comprises means (78)for inhibiting the switching of the receiver means from the sleep stateto the operational state upon detection of a received pulse having awidth that is greater than an upper predetermined limit (T).
 22. Thedevice of claim 14, wherein the receiver means further comprising apredetermined value corresponding to an authorized wake-up signal, saidpredetermined characteristic pattern further comprises a first specificsequence of pulses (80, 82) of uniform amplitude (A) encoding apredetermined binary value forming a wake-up code, and the wake-upcircuit means comprises means (84-100) for indentifying the receivedsignal and decoding therefrom the first signal as a binary value encodedby said received signal first specific sequences of pulses, andcomparing said decoded binary value with said predetermined value. 23.The device of claim 22, wherein the first specific sequence of pulsesfurther comprises a plurality of distinct bursts of pulses, some of saidbursts being a first number of pulses (80) encoding a ‘1’ bit and theother bursts being a second number of pulses (82), different from thefirst number, encoding a ‘0’ bit, and wherein the receiver means furthercomprises an envelope detector (84) delivering as output a series ofbinary values each obtained by integration of a received correspondingburst of pulses, and means (96, 98, 100) for comparing said series ofbits to said predetermined value.
 24. The device of claim 23, whereinthe envelope detector (84) comprises a voltage multiplier circuit (86).25. The device of claim 23, wherein the second number of pulses (80) isdouble the first number of pulses (82).
 26. The device of claim 14wherein said modulated pulse train includes said wake-up pulse precedingsaid plurality of pulses, and the means for demodulating said receivedsignal further comprises means for demodulating said received modulatedpulse train into a received pulse train corresponding to said wake-upsignal and said following plurality of pulses.
 27. An active medicaldevice having wireless communication with a remote active medicaldevice, having a transmitter device, comprising: a first pair ofelectrodes, and a transmitter means (40) for generating modulated pulsetrains comprising a wake-up plurality of pulses in a seriescorresponding to a communication to said remote device, and a wake-upsignal preceding said pulse train, said wake-up signal having apredetermined characteristic pulse pattern, and for applying saidwake-up signal and modulated pulse train pulses said to electrodes (22,24); wherein said predetermined characteristic pattern further comprisesan initial pulse (68) having a high amplitude (A_(w)) corresponding tothe wake-up signal, and wherein said following succession of pulses (70)have lower amplitude (A) than said high amplitude, and wherein saidwireless communication is an intracorporeal communication of electricalpulses conducted by the interstitial tissues of a patient's body. 28.The device of claim 27 wherein said wake-up signal further comprises abiphasic pulse and said modulated pulse train (68, 70, 80, 82) furthercomprises a biphasic pulse train, each biphasic pulse having a positivealternation and a negative alternation.
 29. The device of claim 27,wherein the predetermined characteristic pattern further comprises alatency period (T_(w)) between said initial pulse and the followingsuccession of pulses.
 30. The device of claim 27, wherein saidpredetermined characteristic pattern further comprises a first specificsequence of pulses (80, 82) of uniform amplitude (A) encoding apredetermined binary value forming a wake-up code.
 31. The device ofclaim 30, wherein the first specific sequence of pulses furthercomprises a plurality of distinct bursts of pulses, some of said burstsbeing a first number of pulses (80) encoding a ‘1’ bit and the otherbursts being a second number of pulses (82), different from the firstnumber, encoding a ‘0’ bit.
 32. The device of claim 30, wherein thesecond number of pulses (80) is double the first number of pulses (82).33. The device of claim 27 wherein the device comprises an autonomousleadless capsule.
 34. The device of claim 27 wherein said transmittermeans further comprises means for generating a modulated pulse trainincluding said wake-up signal followed by said plurality of pulses.